Circuit substrates are the building blocks of modern electronic systems, such as computers, cellular telephones, and personal digital assistants. Circuit substrates provide a platform for mounting and interconnecting electronic components, such as integrated circuits, resistors, capacitors, and inductors that perform the electronic functions required in modem electronic systems. Interconnects are conductive structures that connect together the electronic components on a circuit substrate.
Interconnects can be simple structures or complex structures. Simple interconnect structures include conductive traces. Conductive traces are typically thin, narrow strips of conductive material, such as copper, that are formed on a surface of a circuit substrate and connect one electronic component to another. Complex interconnect structures include structures such as vias coupled to pads. Vias are typically cylindrically shaped conductive segments that connect together different layers or components on different layers of a circuit substrate. Pads are typically thin, square conductive structures formed on a surface layer of a circuit substrate. Pads provide sites for connecting components, such as integrated circuits or other electronic devices, to signals available on the circuit substrate. Signals include power or constant potential signals and information carrying signals.
One problem in modem electronic systems is that the systems generate a step demand for current and this step demand for current causes current surges in the conductors that make up an electronic system's power distribution system. These current surges can cause the conductors that make up the power distribution system to emit electromagnetic radiation (sometimes termed electromagnetic interference (EMI)). The amount of radiation that an electronic device is permitted to emit is usually controlled by government regulations. In the United States, the regulations are promulgated and enforced by the Federal Communications Commission. Therefore, the electromagnetic emissions from electronic devices must be controlled.
One solution to the problem of power distribution system electromagnetic emissions requires connecting decoupling capacitors between pads coupled to a constant potential source and pads coupled to a ground or zero potential source. The decoupling capacitors provide a local source of energy, which can quickly be supplied to circuits coupled to the substrate to meet a step increase in demand for current, without causing current surges in other parts of the power distribution system. Unfortunately, the vias that couple the constant potential source to the pads and decoupling capacitors exhibit a large inductance at high frequencies, so a step demand for current in an electronic device can generate voltage drops in the vias. If the step increases in demand for current occur relatively infrequently, then decoupling capacitors work well to reduce unwanted electromagnetic radiation or EMI in an electronic system. However, as the frequency of operation of modem electronic systems increases, the step increases in demand for current will increase in frequency, and this increase in frequency will cause the voltage drops in the vias to generate unwanted noise voltages at a higher rate. Unwanted noise voltages, when generated at this higher rate, create currents which flow in the power distribution system and generate significant amounts of electromagnetic radiation or EMI.
For these and other reasons there is a need for the present invention.